There exist many automated production schemes for controlling the production of liquid products in which electrical power is consumed by a production facility in the production of the liquid products.
An example of such a production facility is a cryogenic air separation plant in which air is compressed and then cooled to a temperature level that is at or near dew point. The compressed and cooled air is then rectified in one or more distillation columns that are commonly designed to fractionate the air into a nitrogen rich product, oxygen and nitrogen rich products or oxygen, nitrogen and argon rich products.
In any air separation plant, a large proportion of the electrical power is consumed in powering the main air compressor used in compressing the air. As such, the electrical power constitutes the major cost of production. In air separation plants that are designed to produce a liquid oxygen product, the liquid oxygen is a value added product in that oxygen constitutes roughly 20 percent of the air to be fractionated. In an air separation plant that is designed to produce a liquid nitrogen product, a nitrogen liquefier is employed that also consumes electrical power.
Air separation plants are provided with a liquid storage capacity that consists of one or more liquid storage tanks that are capable of storing the liquid products produced by the plant. Liquid products to be distributed to meet customer demand may be produced from active production of the facility and/or from stored liquid. Plant operators therefore must decide on how much liquid must be produced by the plant itself to meet such demand as opposed to the amount of liquid to be drawn from storage.
Complicating the decision is that the electrical power costs are a variable factor that can change with the time of day. As indicated above, the cost of electrical power can be the major cost in the production of liquid by a plant and this is particularly true in air separation plants. Additionally, many chemical plants that are designed to produce liquid products, air separation plants in particular, cannot be controlled so that changes in active production are instantaneous. In case of air separation plants, production rates cannot be rapidly swung without changing product purity. Hence, if liquid product is being accumulated or depleted from storage at a particular rate of change and based upon a previous level of demand, any control input into the plant will not be instantaneous and in any case adds a degree of freedom to the problem that makes a decision on the level of plant liquid production particularly difficult.
Hence, a forecast by plant personnel on how much liquid product or products to be produced by a plant is a particularly difficult problem in which often plant production remains stable with production being ramped up and down to meet unusual, high and low demands. This leads to inefficient production on a monetary basis that often results in a plant not capturing its potential profitability.
As will be discussed, the present invention provides a method of controlling a plant or plants that produce one or more liquid products by an expert system controller of control to meet projected customer demand that simultaneously considers projected unit power costs, the amount of liquid within the plant liquid storage capacity and the rate of change within the plant liquid storage capacity. This allows the plant to be more efficiently operated on an economic basis.